Fluorescent dye-containing beads have been used for many years in diagnostics testing, microscope- and flow cytometry-based assays, and combinatorial library synthesis. As such they can be routinely manufactured with a variety of surface modifications which enable many classes of molecules to be coupled and subsequently read and manipulated using commercially available instrumentation. The fluorescent organic dye molecules suffer from a number of disadvantages however including photo-bleaching, different excitation irradiation frequencies and broad emissions. Alternatives to conventional fluorescent materials have therefore been investigated. The substitution of the fluorescent organic molecules with luminescent compound semiconductor nanoparticles or “quantum dots” (QDs) is one approach which is intended to circumvent many of these limitations.
The size of a QD dictates the electronic properties of the material; the band gap energy being inversely proportional to the size of the QDs as a consequence of quantum confinement effects. Different sized QDs may be excited by irradiation with a single wavelength of light to give a discrete fluorescence emission of narrow band width. Further, the large surface area to volume ratio of the QDs has a profound impact upon the physical and chemical properties of the QD.
Nanoparticles that comprise a single semiconductor material usually have modest physical/chemical stability and consequently relatively low fluorescence quantum efficiencies. These low quantum efficiencies arise from non-radiative electron-hole recombinations that occur at defects and dangling bonds at the surface of the nanoparticle.
Core-shell nanoparticles comprise a semiconductor core with a shell material of typically wider band-gap and similar lattice dimensions grown epitaxially on the surface of the core. The shell eliminates defects and dangling bonds from the surface of the core, which confines charge carriers within the core and away from surface states that may function as centers for non-radiative recombination. More recently, the architecture of semiconductor nanoparticles has been further developed to include core/multi-shell nanoparticles in which the core semiconductor material is provided with two or more shell layers to further enhance the physical, chemical and/or optical properties of the nanoparticles. To add further stability, a compositionally graded alloy layer can be grown epitaxially on to the nanoparticle core to alleviate lattice strain between adjacent layers that could otherwise lead to defects and reduce the photoluminescence (PL) emission of the QDs. The emission and absorption properties of the QDs can also be manipulated by doping wide band gap materials with certain metals or luminescence activators to further tune the PL and electroluminescence (EL) at energies even lower than the band gap of the bulk semiconductor material, whereas the quantum size effect can be exploited to tune the excitation energy by varying the size of the QDs without having a significant affect on the energy of the activator-related emission.
The surfaces of core and core/(multi)shell semiconductor nanoparticles often possess highly reactive dangling bonds, which can be passivated by coordination of a suitable ligand, such as an organic ligand compound. The ligand compound is typically either dissolved in an inert solvent or employed as the solvent in the nanoparticle core growth and/or shelling procedures that are used to synthesize the QDs. Either way, the ligand compound chelates the surface of the QD by donating lone pair electrons to the surface metal atoms, which inhibits aggregation of the particles, protects the particle from its surrounding chemical environment, provides electronic stabilization and can impart solubility in relatively non-polar media.